In a typical wireless communications network, devices or wireless devices, also known as mobile stations and/or user equipments (UEs), communicate via a Radio Access Network (RAN) to one or more core networks. The RAN covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g., a radio base station (RBS), which in some networks may also be called, for example, a “NodeB” or “eNodeB”. A cell is a geographical area where radio coverage is provided by the radio base station at a base station site or an antenna site in case the antenna and the radio base station are not collocated. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. Another identity identifying the cell uniquely in the whole wireless communications network is also broadcasted in the cell. One base station may have one or more cells. A cell may be a downlink and/or uplink cell, i.e. a cell for downlink and/or uplink communications. The base stations operate on radio frequencies to communicate over an air interface with UEs within range of the base stations.
A Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the second generation (2G) Global System for Mobile Communications (GSM). The UMTS terrestrial radio access network (UTRAN) is essentially a RAN using wideband code division multiple access (WCDMA) and/or High Speed Packet Access (HSPA) for communication with UEs. In a forum known as the Third Generation Partnership Project (3GPP), telecommunications suppliers propose and agree upon standards for third generation networks and UTRAN specifically, and investigate enhanced data rate and radio capacity. In some versions of the RAN such as e.g. in UMTS, several base stations may be connected, e.g., by landlines or microwave, to a controller node, such as a radio network controller (RNC) or a base station controller (BSC), which supervises and coordinates various activities of the plural base stations connected thereto. The RNCs are typically connected to one or more core networks.
Specifications for the Evolved Packet System (EPS) have been completed within the 3rd Generation Partnership Project (3GPP) and this work continues in the coming 3GPP releases. The EPS comprises the Evolved Universal Terrestrial Radio Access Network (E-UTRAN), also known as the Long Term Evolution (LTE) radio access network, and the Evolved Packet Core (EPC), also known as System Architecture Evolution (SAE) core network. E-UTRAN/LTE is a variant of a 3GPP radio access technology wherein the radio base stations are directly connected to the EPC core network rather than connected via RNCs. In general, in E-UTRAN/LTE the functions of a RNC are distributed between the radio base stations, e.g. eNodeBs in LTE, and the core network. As such, the Radio Access Network (RAN) of an EPS has an essentially “flat” architecture comprising radio base stations that are not controlled by RNCs.
Traditional cellular communication in a wireless communications network or terrestrial radio access network is performed via radio links between devices and radio base stations, e.g. eNodeBs in an LTE system. However, when two UEs or devices are in the vicinity of each other, e.g. within a certain distance range from one another, then direct Device-to-Device, D2D, communication may be considered.
This type of communication between devices may be dependent on synchronization information from either a radio base station or a different kind of radio node e.g. a device. One example of such a different kind of radio node is a cluster head. A cluster head may, for example, be a device with dedicated capability for providing local synchronization information i.e. to act as a synchronization source or to relay synchronization information, or a device with dedicated capability enabling the device to relay synchronization information from a different synchronization source, such as another device, with capability for providing local synchronization information, or a radio base station. The radio node may be capable of relaying or providing synchronization information to at least one device.
Clustering is a well-studied research area for ad-hoc networks. For example, “SURVEY OF CLUSTERING SCHEMES FOR MOBILE AD HOC NETWORKS”, IEEE Communications Surveys & Tutorials, Jane Y. Yu and Peter H. J. Chong, First Quarter 2005 provides a good overview of this field. Note, however, that the discussion here is not only about clustering in ad hoc networking, but about handling whether transmission of synchronization information from a device is activated or deactivated. The device may be out of coverage or in-coverage of other radio base stations and/or radio nodes such as devices with specific capabilities comprising means to act as a synchronization source, relay information from a radio base station or relay information from other devices.
FIG. 1 shows an example of wireless communications network in which synchronization signals for D2D communication between devices or user equipments are broadcasted by radio base stations, e.g. eNodeBs, eNBs, etc. This is shown by the fully drawn, continuous arrows in FIG. 1. Also, in the wireless communications network of FIG. 1, synchronization information (SI) or signals to enable D2D communication between some devices, D1, D2, are broadcasted by a cluster head, CH. This is shown by the dashed-dotted arrows in FIG. 1. Furthermore, in the wireless communications network of FIG. 1, synchronization information or signals for D2D communication between some devices, D3, D4, D5, D6 are also broadcasted by devices having a ProSe-capability. This is shown by the dotted arrows in FIG. 1. The 3GPP has produced a set of requirements for Proximity Service (ProSe) in Technical Report 3GPP TR 22.803 . The goal is to provide local discovery and connectivity to devices in proximity of each other. The ProSe Study Item recommends also supporting D2D operation for out of network (NW) coverage devices.
Here, it may be noted that the lower-left device, D6, in cell 2 receives synchronization information from multiple ProSe capable devices. If such transmissions are performed in a certain manner, it is possible for the receiving device D6 to combine the received signals to enhance reception performance. One such scheme is referred to as a Single-Frequency Network, SFN, transmission, which enable combination of multiple identical sync signals relayed by D2D-enabled devices, D4, D5, within cell 1. Also synchronization information is relayed by devices, D1 and D3, between ‘cell 1’ and the ‘cluster 1’. This synchronization information is in addition to the synchronization information broadcasted by respective eNB. It is also possible that the synch signals are sent via separate configurations and/or at separate time intervals. In such cases, it can still be beneficial to consider both signals, even though the signals cannot be combined. It is enough if at least one of the signals is received properly.
If the cluster head is within network coverage, it may relay the synchronization information to enable devices out-of-coverage to receive this information. The devices D3-D5 in cell 1 are examples of that.
The role of a cluster head, CH, may also be seen as a synchronization information provider to devices in the cluster head's vicinity, e.g. within cluster 1 in FIG. 1. An example of this is further described in the 3GPP standard document R1-134720 “Synchronization Procedures for D2D Discovery and Communication”, where synchronization information is used for aligning a receiver window for receiving transmissions and aligning frequency correction when detecting D2D channels, and for aligning transmitter timing and parameters when transmitting D2D channels.
FIG. 2 shows an example of broadcast of synchronization information, wherein the synchronization (sync) information comprises a synchronization signal (SS) and a synchronization message (SM). These may typically be transmitted with different periodicities. The SSs are transmitted with a sync signal periodicity and the SMs are transmitted with a sync message periodicity. The SSs have a certain sync signal bandwidth and the SMs may have a different bandwidth, a sync message bandwidth. These signals and messages may also be known under other names. One example is Device to Device synchronization signal (D2DSS) for the SS, and Physical Device to Device Shared Channel (PD2DSCH) for the synchronization message.
A device that has detected a synchronization source, for example, a radio base station or a cluster head, may forward such synchronization information by acting as a relay node or synchronization source relay. However, this relay or forwarding of synchronization information from the synchronization source may not be mandatory. When several devices and/or different kinds of radio nodes within a proximity area have capability for providing synchronization information by acting as a synchronization source or synchronizations source relay, there is need for a mechanism for selecting devices for providing synchronization information in a manner that ensures good performance of the wireless communications network.